This invention relates to an induction cooking unit, and more specifically, to a power supply circuit for efficiently energizing an induction coil, causing the same to generate magnetic flux which causes eddy currents to be circulated within the surface of a metallic utensil for heating food.
The principal of induction cooking, that is energizing an induction coil in operative relation to a metallic utensil to circulate eddy currents within the utensil surface for generating heat for cooking food, is not new. As early as 1889, the principal of induction cooking appears to have been known as evidenced by U.S. Pat. No. 400,978 issued to H. F. Watt on Apr. 9, 1889.
Until recently, low frequency alternating current was used to power induction coils for induction cooking. This did not provide satisfactory because of the large conductor requirements and also because of the audible noise associated with the alternate attraction and repulsion of the metallic cooking utensil to the coil. Recent advances have lead to the discovery that the use of high frequency current substantially reduces these problems.
Presently, high frequency induction cooking systems such as thos described and claimed in U.S. Pat. No. 3,781,503 issued to J. D. Harnden, Jr. et al. on Dec. 25, 1973 and assigned to the assignee of the present invention, utilize either series resonant or parallel resonant inverter power circuits to excite the induction coil from a DC source. Such series resonant or parallel resonant inverter power circuits are typically comprised of a resonant circuit formed of the induction coil and a capacitor. The resonant circuit is serially coupled with the gate-anode portion of an SCR or the collector-emitter portion of a power transistor across a DC source. The transistor or SCR is rendered conductive at a frequency of at least 18 kHz to energize the coil with AC.
The series and parallel resonant induction cooking circuits, although both providing good coil excitation each require a complex control scheme for controlling SCR or transistor conduction to regulate coil current. Unless transistor or SCR conduction is well regulated, substantial current is present in the coil when the cooking utensil is removed (defined as the "unloaded condition") because of the substantial increase in the quality factor (Q) of the resonant circuits. This problem is described in further detail in U.S. Pat. No. 4,085,300 issued on Apr. 18, 1978 to R. W. MacKenzie et al.
Additionally, prior art series and parallel resonant inverter power circuits typically require a large filter capacitor coupled across the DC source, which, in turn, increases the cost of the induction cooking unit.
The present invention concerns a simple, low cost induction cooking unit which utilizes a single transistor, non-resonant inverter power circuit, for exciting an induction cooking coil.